1,385 research outputs found
The influence of dispersal on a predator-prey system with two habitats
Dispersal between different habitats influences the dynamics and stability of
populations considerably. Furthermore, these effects depend on the local
interactions of a population with other species. Here, we perform a general and
comprehensive study of the simplest possible system that includes dispersal and
local interactions, namely a 2-patch 2-species system. We evaluate the impact
of dispersal on stability and on the occurrence of bifurcations, including
pattern forming bifurcations that lead to spatial heterogeneity, in 19
different classes of models with the help of the generalized modelling
approach. We find that dispersal often destabilizes equilibria, but it can
stabilize them if it increases population losses. If dispersal is nonrandom,
i.e. if emigration or immigration rates depend on population densities, the
correlation of stability with migration rates is positive in part of the
models. We also find that many systems show all four types of bifurcations and
that antisynchronous oscillations occur mostly with nonrandom dispersal
On the Reliability of LTE Random Access: Performance Bounds for Machine-to-Machine Burst Resolution Time
Random Access Channel (RACH) has been identified as one of the major
bottlenecks for accommodating massive number of machine-to-machine (M2M) users
in LTE networks, especially for the case of burst arrival of connection
requests. As a consequence, the burst resolution problem has sparked a large
number of works in the area, analyzing and optimizing the average performance
of RACH. However, the understanding of what are the probabilistic performance
limits of RACH is still missing. To address this limitation, in the paper, we
investigate the reliability of RACH with access class barring (ACB). We model
RACH as a queuing system, and apply stochastic network calculus to derive
probabilistic performance bounds for burst resolution time, i.e., the worst
case time it takes to connect a burst of M2M devices to the base station. We
illustrate the accuracy of the proposed methodology and its potential
applications in performance assessment and system dimensioning.Comment: Presented at IEEE International Conference on Communications (ICC),
201
Microscopy of a scalable superatom
Strong interactions can amplify quantum effects such that they become
important on macroscopic scales. Controlling these coherently on a single
particle level is essential for the tailored preparation of strongly correlated
quantum systems and opens up new prospects for quantum technologies. Rydberg
atoms offer such strong interactions which lead to extreme nonlinearities in
laser coupled atomic ensembles. As a result, multiple excitation of a
Micrometer sized cloud can be blocked while the light-matter coupling becomes
collectively enhanced. The resulting two-level system, often called
"superatom", is a valuable resource for quantum information, providing a
collective Qubit. Here we report on the preparation of two orders of magnitude
scalable superatoms utilizing the large interaction strength provided by
Rydberg atoms combined with precise control of an ensemble of ultracold atoms
in an optical lattice. The latter is achieved with sub shot noise precision by
local manipulation of a two-dimensional Mott insulator. We microscopically
confirm the superatom picture by in-situ detection of the Rydberg excitations
and observe the characteristic square root scaling of the optical coupling with
the number of atoms. Furthermore, we verify the presence of entanglement in the
prepared states and demonstrate the coherent manipulation of the superatom.
Finally, we investigate the breakdown of the superatom picture when two Rydberg
excitations are present in the system, which leads to dephasing and a loss of
coherence.Comment: 7 pages, 5 figure
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